The present invention relates to a process for applying and forming a coating on an optical element such as an ophthalmic lens or a goggle plate. More specifically, the present invention relates to a process that dip-coats an abrasion-resistant coating on the surfaces of an optical element with more thickness consistency. The present invention also relates to a process to dip-coat a tintable abrasion-resistant coating with more thickness consistency. The present invention further relates to a tinted optical element coated with a tintable coating which is applied with the inventive process.
Ophthalmic lenses made from non-glass materials, such as polycarbonate and CR-39®, become popular due to their low cost and light weight. Polycarbonate provides further advantages such as high refractive index and high impact resistance compared to CR-39®. However, they are more susceptible to surface scratch than mineral glass. Application of an abrasion resistant coating (or hard coating) on the surfaces, both front (convex) and back (concave), of a non-glass ophthalmic lens, is necessary.
Some lens prescriptions need sunscreen function for eye protection. A lens can be made into a sunscreen either during or after the manufacturing. Tinting is the most common way to convert a clear lens into a sunscreen at an optical or dispensary laboratory. It is accomplished by immersing the lens in an aqueous dye bath under a given temperature for a certain amount of time. CR-39.RTM. readily absorbs dye molecules. Thus, lenses made from CR-39.RTM. can be easily tinted to achieve different levels of color and darkness. A hard coating is then applied on the tinted lenses by spin- or dip-coating. On the other hand, polycarbonate doesn't absorb dye as easily. Lenses made from polycarbonate rely on a tintable hard coating, which readily absorbs dye molecules, for its tintability.
The thickness of a tintable hard coating is usually less than about 15 microns. Most tinting dyes can achieve this thickness under normal laboratory tinting conditions. In order to have even color and light transmission, it is desirable to have a coating layer with consistent thickness across the lens surface. The quicker the coating tints, the more important the coating's evenness is.
Lenses made from polycarbonate, either finished or semi-finished, are coated with a hard coating to protect their surfaces from damage. If the coating is tintable, the optical or dispensary laboratory does not need to strip off the hard coating and re-coat with a tintable one. Lenses are commonly dip-coated in the manufacturing site. With the dip coating process, there is a tendency that coating solution flows down due to gravity after the lenses are withdrawn from the coating bath, which makes the thickness gradually increase from the top to the bottom. This is known as a coating “wedge”. This is especially noticeable in the case where the coating solution has very low viscosity (e.g. less than 10 cPs) and the coating process has high optical element withdrawal speed. The coating wedge is also adversely affected by high withdrawal speed of the coated articles from the coating bath.
Although dip coating has long been used as a common coating technique, and its applicability to optical elements such as ophthalmic lenses and goggles is well established, the thickness unevenness has been a reoccurring issue. Many methods aiming at the elimination of the thickness difference along the lens surface have been attempted, such as decreasing the withdrawal speed, flipping the articles 90° after dipping, dashing the articles on an absorbent material like a paper towel, shaking the lenses after dipping, and designing a mechanical blade to wipe off excessive coating solution.
Among the above approaches, it is possible that controlling the withdrawal speed could lead to a more improved uniform coating layer. That is, by decelerating the withdrawal speed during the withdrawal, it may be possible to achieve a more uniform coating layer. However, such an approach would lead to increased processing times for each lens. Moreover, the deceleration profile would need to be very accurately controlled and such control would require expensive control of the coating equipment, making it an impractical option.
Other approaches also require equipment investment to an existing dip-coating line. Despite these investments, results may still remain unsatisfactory.